1. Field of the Invention
The present invention relates to an optical information recording medium, in particular, to an optical information recording medium including a transparent substrate, an optical recording layer absorbing a laser beam formed on the substrate, the optical recording layer containing at least, for example, a light-absorbing substance composed of a dye, and a light-reflective layer composed of materials such as a metal film, wherein writing and reading can be performed with high density and speed using a red laser beam having a wavelength of, for example, 640 to 680 nm.
2. Description of the Related Art
Recently, write-once optical information recording media such as DVD-R and DVD+R have been developed. These media use an organic dye compound corresponding to a semiconductor laser having a wavelength of 640 to 680 nm (for example, 650 to 665 nm) as an optical recording layer. A known CD-R has a track pitch of 1.6 μm and a storage capacity of 600 to 650 MB. In contrast, optical information recording media such as DVD-R and DVD+R have a track pitch of 0.74 μm which is smaller than that of the CD-R, and a storage capacity of 4.7 GB which enables a very high recording density. Furthermore, regarding the recording speed, the CD-R has a reference scanning velocity of 1.2 to 1.4 m/s, whereas the DVD-R/+R has a much higher reference scanning velocity of 3.49 m/s. For recording onto DVD-R/+R, recording instruments having a recording speed of eight times the reference scanning velocity (8× speed) are in practical use. However, at present, a recording instrument having a recording speed sixteen times the reference scanning velocity (16× speed) is not in practical use. In the optical information recording medium using an organic dye compound as the optical recording layer, recording and playback characteristics are required over a wide range of recording speeds, from low speed to high speed. Under the present situation, however, such an optical information recording medium that has recording and playback characteristics adequate to cover a wide range of recording speeds is not realized.
In high speed recording, a high recording power increases the amount of heat generated during recording or per unit time. This causes a problem of thermal strain to become obvious, resulting in variations in the recording pits. Furthermore, the output power of a semiconductor laser is limited. Therefore, a dye material with high sensitivity that can be compatible with high speed recording is desired.
Accordingly, regarding the characteristics of the organic dye material forming the optical recording layer, the complex refractive index (hereinafter referred to as “refractive index”) must be further increased to reduce the thickness of the optical recording layer, thus controlling the heat generated during recording. In addition, the dye in the optical recording layer must be decomposed with a recording power as low as possible. The trend for decreasing the thickness of the optical recording layer is, when the recording speed is represented based on a reference scanning velocity (corresponding to 1× speed) of 3.49 m/s (about 3.5 m/s), the average of 60 nm at 2× speed, 54 nm at 4× speed, 44 to 47 nm at 8× speed, and about 40 nm at 16× speed etc. However, even if the molecules of the organic dye material are designed to have a high refractive index, possible chemical structures are limited. Therefore, under the present situation, such a dye that can be compatible with high speed recording cannot sufficiently be designed. On the other hand, the recording power at low speed recording is too low. Therefore, when the thickness of the optical recording layer is decreased in order to provide characteristics for high speed recording, it is difficult to form sufficiently large pits.
As described above and will also be described later, in high speed recording, at the recording speed of 8× or more in particular, the amount of heat generated during recording is increased because of the high recording power. Accordingly, configuration factors and thermophysical properties must be designed so as to be suitable for thermal control during recording. The configuration factors include the shape of the optical recording layer in a pregroove on a substrate, the shape of the light reflective layer in the pregroove, and the shape of the substrate. The thermophysical properties include the decomposition temperature of the dye and the calorific value. In order to decrease the recording power, optical design of the dye, for example, increasing light absorption of the organic dye material is also required. In a media design suitable for high speed recording, the thickness of the optical recording layer is reduced. As a result, the physical size of the pits formed is decreased in low speed recording. In order to perform appropriate recording from low speed to high speed, the pits must be made optically larger using an organic dye material having high refractive index. Furthermore, a measure for heat dissipation in high speed recording is also required.
As described above, in order to perform the appropriate recording from low speed to high speed, in particular, recording at high speed recording, the configuration factors and thermophysical properties of the dye are preferable to be designed so as to be suitable for thermal control and the heat dissipation measure (see, domestic re-publication of PCT international publication for patent application No. 2001-59778).